Gravity meter



March 24, 1942.

D. H. CLEWELL GRAVITY METER Filed Feb. 21, 1940 IN: I

Patented Mar. 24, 1942 s PATENT OFFICE GRAVIT'I METER.

Dayton H. Clewe'll, Dallas, Tex.,.assignor, by mesne assignments, toSocony-Vaouum Oil Company, Incorporated, New York, N. Y., a corporationof New York Application February 21, 1940, Serial No. 320,054

I 2 Claims. (Cl. 265-1.!)

This invention relates to improvements in gravity meters and moreparticularly to an improvement in that type of gravity meter in which amass is resiliently supported from a fixed support and in a verydelicately balanced state during its operation and is clamped rigidly inposition at other times so that the shock incident to moving it aboutwill not injure it or change its calibration. In particular, the presentinvention is directed to providing in such a gravity meter a means toreduce the oscillation that is normally occasioned by the release of theclamping means or by some outside disturbance.

It is well-known that gravity meters are extremely sensitive instrumentswhich must be protected against shock or disturbances of any kind. butit is nevertheless necessary to move these instruments from place toplace if they are to be used in geophysical exploration, in order thatmeasurements may be made at these different locations. In order thatthis may be done without injury to the delicate mechanisms or destroyingthe calibration of the instrument, it has been common to provide theinstrument with clamps that clamp the mass in position duringtransportation, and until the time comes to take a reading. At that timethe clamps are released and the mass allowed to come to rest at aposition of balance that is indicative oi the relative gravitationalforce being exerted at that particular location. However', the processof coming to rest almost invariably takes a very considerable 7 periodof time and often places undesirable strains in the instrument whichstretch the supporting wires or in other ways affect its calibration. Asthe mass of this instrument is in such a delicately balanced state withrespect to the torsional elements which support the mass, it

'cannot be moved or displaced very far from its original position and itis therefore necessary to keep this mass within as small a range ofdisplacement as possible.

Regardless of how carefully the clamps are made that clamp the mass inposition, it is almost impossible to release the mass from them withoutimparting to the mass animpetus that starts it swinging on its resilientsupport. Being in delicate balance, unless there are damping meansprovided, the swinging continues for a long period of time. Severaltypes of damping means have been suggested but are generallyunsatisfactory, because they not only damp the oscillations of the mass,but they so restrict its movement as to greatly reduce the accuracy ofthe measurement.

5o. amount or force applied which is estimated as It is well-known tothose skilled in the art that better than 50 per cent of the timeconsumed in taking measurements at each station in the field whileconducting gravimetric surveys is attributed to the indefinite period oftime required for the oscillating mass to naturally come to rest.

According to the present invention, a new type of damping or centeringmeans is provided which quickly and simply helps to bring the mass toits zero or base position, and yet does this without interiering in anyway with the ultimate sensitivity of the device. I

This is accomplished by intermittently ing a force to a suspended massin opposition to the movement oi the mass during its oscillation. Therepulsive force is applied by an induction coil positioned adjacent tothe mass and actuated by a charged condenser in series therewith and theforce exists only so long as the transient current from the condenserflows. Since the charge on the condenser dissipates itself through thecircuit very rapidly, the repulsive force is applied ior only a shortperiod of time and may be regarded as only an impulse. After this forcehas beenapplied, the condenser is recharged and the application of forcerepeated until the mass comes to a zero or dead heat position.

In the preferred embodiment only one coil is used and this is positionedadjacent to one end of the mass, although it is quite possible andwithin the concept of the invention to position a similar circuit on theopposite side oi! the suspended mass and accomplish the same result.

Generally, the charging potential of the condenser will be applied bymeans of a battery or other source of direct current through a simplepush button switch.

Insome of the newer gravity meters arrangements are beingmade so thatall readings are taken with the mass in a fixed or base position, thedifference in gravitational force being compensated by bringing anadditional force to bear on the mass to hold it in the base position.This may be done electromagnetically or electrostaticaliy or by othermeans which do not form a part of this invention and will not bediscussed here. However, when such is the case, the present inventionhas a particular application in that the. mass can be immediatelycentered and an being sufilcient to hold the mass in its centeredposition. It on observation this force applied is not sufllcient tomaintain the massin its centered position, further adjustment in theforce applied to the mass can be made and the procedure repplyture oithe device may be made all within the scope of this invention as may bereadilyunderstood by those skilled in the art.

In the drawing the sole figure is a perspective view of a part of agravity meter-embodying the principle 01 this device.

As shown in the drawing, a mass i is supported by torsional elements 2and 3 from the support 4 and held in place when not in use by clampingmembers 5, 6, l, and 8. An optical system not shown may be arranged inany suitable relation to the mass so that its position can be accuratelydetermined at any time and its movement during its oscillation can befollowed simply by observation through the optical system. On the sideof the mass acUacent to an extremity thereof is attached an electricalconductive plate I such as copper or aluminum, and a coil H) whichconsists of a number of turns of copper wire is disposed adjacent theplate 9 in such a manner i3, this force being always applied inopposition to the movement of the mass during its oscillation. In thismanner an impulse may be applied to the mass and the result of thisimpulse viewed through the optical system. Further impulses are thenapplied until the mass is in its centered or "dead beat position. 4

While the preferred embodiment as shown by the drawing includes only oneinductive circuit disposed adjacent to one end of the mass, verysatisfactory results may be obtained by placing another inductivecircuit on the other side of the mass and directly opposite the coil IIIto be operthat its axis is perpendicular to the plate 9.. The

'switch I3 is manually operated to close the circult connecting the coili0 and the condenser I l. Immediately upon closing the switch theinductance of coil I II and capacity of condenser Ii enable the chargeon condenser ii to produce a short train of electrical currentoscillations in coil II. This alternating current in coil iii induceseddy currents in plate 8 so that a repulsive force is exerted .betweencoil l0 and plate 9. The switch" is so'arranged that when it is releasedbattery I! is automatically connected to recharge condenser H.

In operation the clamping member parts 5. 6, I, and 8 are released andthe oscillation of the mass which results from their release is observedthrough an optical system. By observing through the optical system themovement of the suspended mass, a repulsive force may be applied to themass by manual operation of the push button switch ated alternately tospeedup the centering of the oscillating mass. Numerous other changeswill be immediately apparent to those skilled in the art and it is to beunderstood that the embodiment shown is only one possible embodimentwhich has been found particularly advantage-- ous and that many otherembodiments may be made all within the scope of the appended claims.

I claim:

1. In a gravity meter in which a mass is resiliently suspended from asupport during the measuring process, the improvement that comprisesmeans fpr producing a damping impulse, said means including an inductivecoil fixed to the support, an electrical capacitance, means forelectrically charging the capacitance, means for discharging capacitancethrough inductive coil, and an electro-conductive non-magnetic ele mentattached to the mass adjacent the inductive coil whereby a discharge ofthe capacitance through the inductive coil will produce a momentaryrepulsion force between the mass and the inductive coil.

2. In a gravity meter in which a mass is re siliently suspended from a.support during the measuring process, the improvement that comprisesmeans'for generating a damping impulse, said means including aninductive coil fixed to the support opposite a movable portion of themass with the axis of said inductive coil being parallel to thedirection of motion of said movable portion of the mass, a non-magneticelectrical conducting material fixed to the mass at a point near theinductive coil, an electrical capacitance, a manually operable switchand a.- bat tery, all electrically connected together in such mannerthat the normal rest position of the switch enables the battery tomaintain the capacitance in a charged condition until operation of theswitch discharges the capacitance through the inductive coil therebyproducing a short damped series of current oscillations which induceeddy currents in said conductive memher, said eddy currentsreacting onthe current in the coil to cause a momentary force to repel the massfrom the inductive coil.

DAYTON H. CLEWELL.

